1. Field of the Invention
The present invention relates to a power supply and a power management method thereof, and more particularly to a hybrid power supply and a power management method thereof.
2. Description of the Related Art
Fuel cell batteries have been identified as a clean, highly efficient means of energy storage and a means for converting energy to useable electricity. Methanol fuel cells in particular have been identified not only as a means of storing energy and converting stored energy to electricity but also as part of a renewable (as opposed to exhaustible) energy system based on methanol. When the fuel cell battery outputs at maximum power, however, the low conversion efficiency of the fuel cell battery decreases the numbers of hour fuel cell battery can be used. Thus, a hybrid power supply comprising a fuel cell battery is desirable.
FIG. 1 is a schematic diagram of a conventional hybrid power supply 10. In FIG. 1, fuel cell battery 11 provides electric energy to a DC-DC converter 12. An output voltage of an operational amplifier 16, determined by a divide voltage V of the rechargeable battery 13 and a reference voltage Vref, is transmitted to a control unit 15. The control unit 15 transmits a control signal 17 based on the output voltage of an operational amplifier 16 to the DC-DC converter 12 for adjusting the output voltage Vout of DC-DC converter 12. When the maximum power of the fuel cell battery 11 is not sufficient to power the DC-DC converter 12 to provide sufficient power to the loader 14, the voltage of the fuel cell battery 11 decreases until power failure occurs. To avoid power failure, a power management method for the hybrid power supply 10 is desirable.
FIG. 2 is a schematic diagram of a conventional hybrid power supply of U.S. Pat. No. 6,590,370. A status value of the fuel cell battery 21 and a reference voltage Vref are used to generate a feedback signal for adjusting the electric energy derived by the DC-DC converter 22 from the fuel cell battery 21. Thus, the conversion efficiency of the fuel cell battery 21 increases and the probability of power failure decreases. The output current of the fuel cell battery 21 is affected by the concentration of the fuel and the temperature of the fuel cell battery 21. In this embodiment, a feedback mechanism and a rechargeable battery 23 are applied to control the current derived by the DC-DC converter 22 from the fuel cell battery 21 to maintain the output voltage of the fuel cell battery 21. When the rechargeable battery 23 is at full charge or the voltage of the rechargeable battery 23 is equal to a cutoff voltage, however, the rechargeable battery 23 can no longer adjust the fuel cell battery 21 and may damage the rechargeable battery 23.
FIG. 3 is a schematic diagram of a conventional hybrid power supply of U.S. Pat. No. 6,590,370. A control signal VCONTROL is generated from the operational amplifier 32 receiving an output voltage of fuel cell battery 31 and a reference voltage VREF. The DC-DC converter 33 is a boost-type DC-DC power converter, wherein in this embodiment, the DC-DC converter 33 is made of a MAXIM chip (chip number: MAX1701) and other required elements. When the rechargeable battery 36 is at full charge, the rechargeable battery 36 might be damaged if the fuel cell battery 31 is still outputting the power to the rechargeable battery 36. Thus, the U.S. Pat. No. 6,590,370 provides a protection circuit 35 to protect the rechargeable battery. When the voltage of the rechargeable battery 36 is equal to a charge cutoff voltage, i.e. the secondary is at full charge, only a few volts are transmitted to the rechargeable battery 36 and the remaining voltage is transmitted to the protection circuit 35. The protection circuit 35 converts the remaining voltage into heat for diffusion. Although, the problem of the rechargeable battery 36 is solved, the wasted energy of the fuel cell battery 31 and heat generated by the protection circuit causes new issues. Furthermore, when the power stored in the rechargeable battery 36 is exhausted, the hybrid power supply fails.